Before, when we receive an olm-encrypted event and do not yet have the sender's identity keys, we are not able to verify the decrypted event, causing a decryption error. Matrix v1.15 adds a mechanism to send device keys info along with the encrypted message. With this, we can fix some situations where such decryption cannot be performed properly. A whole fix still needs to properly query the keys for these undecryptable sessions.

Type: add

CCBUG: https://iron.lily-is.land/T141

Log in to a new session using kazv and verify E2EE still works.
Verify unit tests pass.
Verify unit tests cover the checks in https://spec.matrix.org/v1.18/client-server-api/#validation-of-incoming-decrypted-events .